Massive multiple-input multiple-output (Massive MIMO) is a well-known key technology of 5G (the 5th generation mobile communication) in the industry. A system capacity in the Massive MIMO technology is increased because of utilization of a high spatial degree of freedom in the Massive MIMO technology, so that multi-user spatial multiplexing can be performed to obtain a gain. To obtain a higher gain of spatial multiplexing, a network device needs to obtain accurate channel state information (CSI).
Currently, a method for obtaining CSI by a network device includes: sending, by the network device, a channel state information—reference signal (CSI-RS) to a terminal device; and performing, by the terminal device, channel estimation based on the CSI-RS sent by the network device, selecting a precoding matrix from a stored codebook based on an estimation result, and feeding back an index of the selected precoding matrix in the codebook to the network device over an uplink channel, where the index is denoted as a precoding matrix indicator (PMI). The precoding matrix selected by the terminal device is used to represent the CSI.
This mechanism limits accuracy of the CSI represented by the precoding matrix, and consequently, the network device cannot obtain accurate CSI.